We investigated effect of reducing thickness of TiN buffer layers on growth of the smooth GaN layers. The sputtered TiN layers with thicknesses in the range of 2 to 100 nm were deposited on sapphire substrates. The sputtered TiN layers were exposed NH3 + H2 mixed gas atmosphere at about 1000°C to enrich nitrogen concentration of the layers. GaN layers were deposited on the nitrogen-enriched TiN layer using a MOCVD method. Average grain size of the nitrogen-enriched TiN layer was minimized at the thickness of 5 nm. In the initial stages of GaN growth, density of GaN hexagons grown on the 5nm-thick TiN layers was the highest. The 2μm-thick GaN layers grown on the 5nm-thick TiN layers exhibited the smoothest surface. Thus, the 5nm thickness is believed to be the best thickness for the smooth GaN growth on the sapphire/TiN substrates.